For current optical recording media runlength limited codes are used. For example, the Digital Versatile Disc (DVD) features a channel modulation which ensures a limitation of the runlengths of ones and zeroes of the recovered data stream basically between 3 and 11. Synchronisation patterns are modulated to get a runlength of 14. Therefore, these patterns can easily be distinguished from the other data.
More recent high density optical recording media, which are based on blue laser optics, will presumably use an even shorter runlength range of 2 to 8 consecutive ones or zeroes. An example for such a recording medium is the so called Blu-Ray Disk(BD). Unfortunately, the modulation transfer function (MTF) of the corresponding optical channel causes a typical amplitude of 2T runlengths, which is −26 dB smaller than the amplitude of the longest runlengths, as shown in FIG. 1. Therefore, additional signal processing is required to guarantee an acceptable Bit Error Rate (BER).
In current applications for optical recording media and hard disks partial response maximum likelihood detectors (PRML, Viterbi Decoder) are employed in the read channel prior to the demodulation process, in order to cope with higher rates of intersymbol interference (ISI) and to obtain an improved noise margin. A state of the art read channel is depicted in FIG. 2. Due to the modulation of the optical recording data pattern certain restrictions apply to the processing of such signals. For example, signal patterns with short runlengths are at the edge of the channels passband and exhibit a reduced signal magnitude. They are, therefore, vulnerable to bit misdetection. In addition, due to filter ripple and transition noise longer runlengths, which are actually detectable by simple slicing, might fail to be detected by a maximum runlength decoder. One further problem associated with the detection of bit information is the signal asymmetry, which disturbs the process of data slicing. This asymmetry has a strong influence on the quality of the bit detection, especially for the shortest runlengths.
It is, therefore, an object of the invention to propose a method for bit recovery in an asymmetric data channel, which overcomes the above problems, and an apparatus for reading from and/or writing to recording media using such method. Though the invention is described in the following with reference to optical recording media, it is also applicable to other types of recording media where signal asymmetry occurs.